Flexible base materials for printed circuits or flat electrical cables (hereinafter referred to as flexible base materials) are usually prepared by bonding an electrical conductor and an organic polymer insulator with an adhesive. However, a thermal hysteresis from contact bonding under heat or the like during the bonding operation has posed a number of problems. One of such problems is that a base material while cooling curls, twists, or wraps because of the difference in linear expansion coefficient between the conductor and the insulator, which seriously interferes with the subsequent conductor patterning. Another is the lowering of flame retardancy caused by the presence of an adhesive layer between the conductor and the insulator. Still another problem is a high production cost of flexible base materials on account of polyimide films in use being costly and the bonding operation time-consuming.
A number of adhesive-free method have been devised in order to solve the problems arising from the use of adhesives. For example, Japanese Kokai Document No. 81-94,689 proposes to coat a conductor directly with an organic polymer solution. These methods have indeed solved the problem of the adhesive-related lowering of flame retardancy, but still left unsolved the problems of occurrence of curling from the difference in linear expansion coefficient between the conductor insulator and the subsequent interference with the conductor pattering.
Japanese Kokai Document No. 81-23,791 consequently proposes to coat a metal foil with a solution of polyamideimides and apply a heat treatment to correct the curling after drying caused by the difference in linear expansion coefficient. This method, however, is also time-consuming and poor in productivity.
Other methods disclosed in Japanese Kokai Document Nos. 85-157,286 and 85-243,120 propose to make flexible base materials of minimal curling by forming resins of low thermal expansion on a conductor from a solution of polyimides or their precursors of a specific structure. The resin films formed on the conductor, however, show some shortcomings in properties, particularly in flexibility to withstand bending, and flexible printed circuits manufactured therefrom show too large thermal shrinkage and insufficient dimensional stability when immersed in a soldering bath.
An object of this invention is to provide a method of making commercially useful flexible base materials which have as little differences as possible in linear expansion coefficient between a conductor and an insulator, do not curl, twist or wrap on application of a thermal hysteresis, and possess sufficient adhesive strength, flexibility, and dimensional stability.
The present inventors studied the aforesaid problems from various angles, found that the use of polyamideimides of a specific structure as insulator yields commercially useful flexible base materials which do not curl, twist, or wrap with a change in temperature and possess good flexibility and small thermal shrinkage, and completed this invention.